1. Field of the Invention
The present invention relates to en electron beam irradiating apparatus and an electron beam irradiating method.
2. Related Art
The technique of forming a fine pattern on a semiconductor substrate by using an electron beam is widely used for forming fine semiconductor structures and drawing mask patterns for optical lithography.
Typically, in conventional electron beam irradiating apparatuses, an electron beam emitted from an electron source is condensed by a condenser lens, and applied to a first shaping aperture. The electron beam passed through the first shaping aperture has a rectangular opening. A second shaping aperture is placed on a downstream side of the first shaping aperture. A projection lens and a shaping deflector are placed between the first and second shaping apertures. The projection lens is excited so as to form an image of the first shaping aperture on the second shaping aperture.
The shaping deflector is provided to change the position of the image of the first shaping aperture on the second shaping aperture. The shaping deflector shapes the cross section of the electron beam transmitted by the second shaping aperture to form a rectangle or a right-angled triangle having an arbitrary size by adjusting overlap between the image of the first shaping aperture and the second shaping aperture.
An object lens and an object deflector are placed on a downstream side of the second shaping aperture. A sample on which a pattern should be drawn is placed on a downstream side of them. The object lens forms a reduction image of the second shaping aperture. The object deflector changes the image forming position on the sample. A blanking deflector and a blanking aperture are placed between the condenser lens and the first shaping aperture. Control as to whether to apply the electron beam to the sample is exercised by applying an electric field to the blanking deflector and intercepting the electron beam with the blanking aperture.
When an electron beam propagates in the space, scattering of electrons is caused by interaction between electrons. It is known that this widens the energy distribution of the electron beam. As the beam current increases, the spread becomes larger, resulting in an increase in chromatic aberration at lenses. The beam current emitted from the electron gun has a value of, for example, approximately 100 μA, and the beam current transmitted by the first shaping aperture has a value of approximately 3 μA. For preventing the increase in chromatic aberration, therefore, it is desirable to shorten a portion in which the beam current is large, i.e., the distance between the electron gun and the first shaping aperture. In the case where the blanking deflector is placed on the upstream side of the first shaping aperture, however, there is a limit in the distance shortening.
On the other hand, an example in which the blanking deflector is installed between the first shaping aperture and the second shaping aperture is shown in Japanese Patent Application Laid-Open Publication No. 8-316128. In this example, the image of the first shaping aperture on the second shaping aperture moves at the time of the blanking operation, resulting in a problem that the current distribution on the sample changes.
If the blanking deflector is placed on the upstream side than the first shaping aperture in the conventional electron beam irradiating apparatus, it becomes necessary to lengthen the portion in the barrel through which a large current beam flows, resulting in a problem of increased chromatic aberration at the lenses. If the blanking deflector is placed on the downstream side than the first shaping aperture, there is a problem that the current distribution on the sample surface changes at the time of blanking operation.